Pediatric M-Mode Z-Scores

(Note: the calculator is now HERE )

I finished off what I thought were the more useful z-score calculations based on data from the article "Normal values of M mode echocardiographic measurements of more than 2000 healthy infants and children in central Europe" (link). Included in this calculator are z-scores and normal ranges for:

• right ventricle (RVDD)
• ventricular septum (IVS)
• left ventricle end diastolic dimension (LVEDD)
• left ventricle end systolic dimension (LVESD)
• posterior wall (LVPW)
• left atrial diameter (LAD)

I excluded calculations of the RV free wall, wall thickness in systole, and the arterial diameters- in part because I am not routinely reporting these measurements, but also because I question the application (PA diameter by m-mode??). Also, I hesitate to include the calculation of the left atrial z-score because reporting left atrial volumes is so much more descriptive, but alas I am not prepared to calculate LA volume z-scores for pediatrics...

I tried to use the published regression equations, both for the sake of simplifying the calculation and for the "continuously variable" effect, but ultimately abandoned this approach for several reasons:

• The regression often did not yield the same result as the tabular data. For instance, using the formula for LVEDD for a child with a BSA of 0.5 m² predicts a diameter of 28.19 mm; using the published table, the value is shown as 29.0 mm. The IVS predicted by the equation for patients with BSA's of 0.25 m2 and 2.0 m2  is 3.48 mm and 7.5 mm respectively; the same data from the tables is 3.8 mm and 9.3 mm.
• I had to build a "lookup" routine for the standard deviations. As the authors did not publish prediction equations for the standard deviations, the values had to be hashed from a table anyhow. Adding another lookup table for the mean values was not much more work.
• Infants were grouped by weight rather than BSA. Because, according to the authors "body surface area changes only minimally." The authors do not explore this topic any further though it begs the question of how the correlations were affected, i.e., what was the correlation coefficient when the infant's BSA was included in the analysis, and how much was it improved by removing them from the analysis? In part, the decision to break the infants out of the BSA relationship probably stems from a lack of understanding of the underlying relationship, as was recently and elegantly described by Sluysmans and Colan. Anyhow, because they grouped infants in this manner, z-scores could not be calculated using their prediction equations unless I mixed and matched the techniques (predict the mean using the BSA-based equation, and use tables to lookup the weight-based standard deviations).

In summation, this calculator simply addresses the published tables and performs lookup routines for the mean and standard deviation based on the appropriate index: weight for infants 2-4kg, BSA for subjects with BSA >0.25.

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Update 6/2008: I re-worked the calculator to be quicker and prettier...

M-Mode Z-Score Calculator

Ebstein's Anomaly: Measuring the Spectrum

Ebstein's anomaly is a malformation of the tricuspid valve and right ventricle. The classic description is something along the lines of: a tethered and displaced septal (and posterior) leaflet, a "sail-like" anterior leaflet, and a variable "atrialized" portion of the right ventricle. Echocardiographically, it is important to understand the measurable parameters that help describe the spectrum of disease, and thus help guide the management.

At one end of the spectrum is a tricuspid valve that is apically displaced just slightly more than normal:

Tricuspid Valve Offset Calculator:

At the other end of the spectrum are the critically ill neonates whose biventricular circulation— and very survival— are not certain:

Great Ormond Street Ebstein Index Calculator:

Ultimately, it is the actual leaflet and right ventricle morphology which dictates just how much and what kind of repair can be attempted, and 3D echo seems best suited to describing these.

Source: Ebstein's Anomaly
Attenhofer Jost et al. Circulation.2007; 115: 277-285

Fetal Echo Z-Scores: the SGA conundrum

The fetal echo z-score calculator was initially a proof-of-concept project (as were all of the calculators at ParameterZ.com). It has since proven itself to be quite useful, and I refer to it for nearly all fetal echos now.

However, a shortcoming of the current calculator is highlighted by this common referral:

• SGA (Small for Gestational Age)
• RV > LV
• LV measures lower limits of normal for EGA

The real difficulty here of course, is the SGA baby. Based purely on dates, the baby is known to be small- and all measures of her cardiac structures are sure to be small as well.
Absolutely.
Small.

But can't her heart still be relatively normal ? 

In the article Development of Z-scores for fetal cardiac dimensions from echocardiography, regressions are presented based on three independent variables: femur length, biparietal diameter, and EGA. According to the authors, "femur length gave the better correlation coefficient with fetal cardiac dimensions" though admittedly, all the independent variable gave good correlations. And a good discussion is made over EGA as a surrogate for fetal size, the importance of considering fetal size, fetal growth and fetal size, fetal size, fetal size, ...

So why don't I build the calculator based on the femur length- ergo the fetal size?

Because: I have now become an advocate of measuring the fetus:

• measuring the BPD
• measuring the head circumference
  
• measuring the abdominal circumference
• measuring the femur length

Now, I measure the fetal size and, taking advantage of the OB calculation package on the ultrasound machine, use the size-determined EGA- rather than the date-determined EGA.

 

The new and improved fetal echo worksheet allows you to edit the EGA and automatically update all of the z-scores based on your measurements. And, just for fun, makes estimations of the EDC and LMP.

ErratumZ

Users of the Pediatric Cardiac Valve Z-Score calculator might be interested to know about an erratum to the reference article:

The tricuspid valve nomograms have been re-published.

In the process of refining the calculator on ParameterZ.com, it was noted that tricuspid valve z-scores calculated by the supplied formula differed considerably from the result when using the nomogram. I contacted the lead author, he contacted their statistician, and it was discovered that the original nomograms were in error.

Fortunately, the error involved only the nomograms themselves- the formulae are (still) correct and thus, none of our calculations were affected.

Relatively Normal: pediatric echo z-scores

Big people have big hearts. Little people have little hearts- and mitral valves, aortas, and pulmonary arteries. As a student sonographer, one of the few things I excelled at was committing things to memory. I was all about flashcards. I had flashcards for everything, but particularly for normal dimensions. Normal left atrium? 19-40mm. Normal IVS? 6-11. LV? 37-56. In the adult echo arena this knowledge served me well as part of a routine:

• Perform study
• Obtain standardized measurements
• Compare measurements to established criteria

Executing these simple steps allowed me to declare some exams within normal limits and others, unequivocally abnormal. I basked in the power and the glory. But pediatric echo brings a whole new set of flashcards. There are all the same adult values, and then again several times over: for kids with BSA up to 0.5 m2, 0.5 to 1.0 m2, 1-1.5, and the babies, and preemies, and what about the transverse arch... Yikes. Too many flashcards.

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The problem of relating normal values over a wide variety of body sizes is at least as old as pediatric cardiology itself. Knowing if a newborn's tricuspid valve is too small or a two year-old's coronary artery is too big, is essential to the modern practice of pediatric echo.Enter the nomogram: Found in the backs of textbooks and throughout the literature, nomograms soon became the staple that replaced my flashcards. Now, all I needed was a copy machine and notebook. Nomograms aren't perfect, however. If the nomogram is printed too small (as I have deviously done with the above sample) it is quite difficult to resolve small differences between our measurement and the printed reference- and almost all of them require some interpolation on our part. Still, for the most part, the power and the glory had returned. Soon however, the absurdity of sitting in front of a computer/echo reading station, while interpolating hash-marks in a notebook caught up with me. This is the Age of Information? Besides- what if an unscrupulous cardiology fellow absconded with our Precious Notebook of Nomograms? What if?

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Underlying each nomogram is the theory that a predictable relationship exists between the independent measure (age, weight, BSA, etc.) and a dependent variable (coronary artery diameter, annulus dimension, etc.). Further underlying each of these relationships, is the assumption that these variables have a normal distribution in our population.When we perform an echocardiogram and measure, for instance, the left coronary artery, and then ask "is it normal?"-- what we are really asking is: "how does our measurement compare to the mean of the population of other (normal) humans of this size?" The answer is best given with one number: the z-score. The z-score tells us in one simple, elegant number how our measure relates to the population.

• Exactly normal: z = 0
• pretty much normal: ± 1
• too small: -3
• gigantic: +7

Provided the authors of all these nomograms have actually published the predictions equations themselves (and not just the nomograms), we can now extract the information, leverage it to construct the predicted mean and standard deviation for our measure, and report a z-score. No more flashcards. No more notebooks. Power. Glory.

ParameterZ.com